Numerous processes are known for the modulation of visible light. Among these processes, electrochromic techniques use the reversible change of color and/or optical density obtained by an electrochemical redox reaction of an electrochromic material in which the oxidized form and reduced form are of different colors and/or different optical densities.
Electrochromic materials change their optical properties due to the action of an electric field and can be changed back to the original state by a field reversal. In most electrochromic materials, the mechanism underlying the optical change is the insertion of ions into the electrochromic material and the subsequent extraction of the same ions. The devices can show open circuit memory, i.e., voltage has to be applied only when the optical properties are to be altered. Most electrochromic devices require an ion-containing material (electrolyte) in proximity with the electrochromic layer as well as transparent layers for setting up a distributed electric field.
Recently it has become evident that electrochromism occurs in numerous transition metal oxides and organic materials [See C.M. Lampert and C.G. Granqvist, "Introduction to Chromogenics", Large-Area Chromogenics: Materials and Devices for Transmittance Control, SPIE Institute Series Vol. IS 4, pp 2-19 (1990)].
Many applications exist for electrochromic materials including display panels, variable transmittance windows and variable reflectance mirrors. Prior art electrochromic materials include metal oxides such as WO.sub.3, MO.sub.3, V.sub.2 O.sub.5, Ir.sub.2 O.sub.3 and Nb.sub.2 O.sub.5, polymers such as polyaniline, polyacetylene, polypyrrole and polythiophene, and aqueous solutions of metal ions such as Zn.
U.S. Pat. No. 4,009,936 issued Mar. 1, 1977 to Kasai discloses electrochromic display devices including a solid electrochromic material and a solid electrolyte. The electrochromic material is selected from tungsten oxide, molybdenum oxide, titanium oxide, vanadium oxide, cobalt tungstate, tin oxide, tellurium oxide, iron oxide, rare earth oxides, metal halides, strontium titanate, metal carbonyls, salicylidene aniline, and organic materials containing a hydrazone group, an osazone group, a semicarbazone group or a sydnone group. The electrolyte is selected from Ag.sub.7 I.sub.4 PO.sub.4, AgI, and AgI in combination with a member of the Ag.sub.4 P.sub.2 O.sub.7 series, the Ag.sub.2 WO.sub.4 series, the RbI series, the NH.sub.4 I series, the KCN series, or the C.sub.4 H.sub.8 SCH.sub.3 I series. The display devices also include a transparent electrode in contact with the electrochromic material comprising a conductive film of unspecified composition coated on a glass substrate. The devices are said to be useful for display purposes, e.g., timepieces and the like.
U.S. Pat. No. 4,448,493 issued May 15, 1984 to Matsudaira et al discloses electrochromic display devices including an electrochromic layer and a solid proton conductive layer. The electrochromic layer consists of a transition metal oxide such as WO.sub.3, MoO.sub.3, TiO.sub.2, Ir.sub.2 O.sub.3, Rh.sub.2 O.sub.3, NiO or V.sub.2 O.sub.5. The proton conductive layer comprises a mixture of acids selected from titanic, stannic, antimonic, zirconic, niobic, tantalic and silicic acid. The display devices also include a transparent electrode contacting the electrochromic material and comprising a thin film of indium oxide (In.sub.2 O.sub.3) or tin oxide (SnO.sub.2) deposited on a transparent substrate such as glass or synthetic resin. The disclosed devices are said to possess shortened response times, with speeds on the order of 1 to 10 seconds being exemplary.
U.S. Pat. No. 4,459,035 issued Jul. 10, 1984 to Nanya et al discloses electrochromic display devices including a reduction electrochromic material and an oxidation electrochromic material separated by an ion permeable insulating layer. The reduction electrochromic material is WO.sub.3 or MoO.sub.3, while the oxidation electrochromic material is iridium hydroxide [Ir(OH).sub.n ], rhodium hydroxide [Rh(OH).sub.n ] or nickel hydroxide [Ni(OH).sub.n ]. As the ion permeable insulator, Ta.sub.2 O.sub.5, Cr.sub.2 O.sub.3 or SiO.sub.2 may be used. The display devices also include a transparent electrode contacting the reduction electrochromic material and a counter electrode contacting the oxidation electrochromic material. During operation of the device, the oxidation electrochromic material is said to function as an acceptor of protons, thereby preventing evolution of hydrogen gas on the surface of the counter electrode. The devices may be used as display panels for electronic timepieces.
U.S. Pat. No. 4,233,339 issued Nov. 11, 1980 to Leibowitz et al discloses electrochromic display devices including an electrochromic material and an electrolyte. The electrochromic material may comprise WO.sub.3 which has been partially converted from the amorphous to the crystalline form which is said to significantly increase the etch resistance of the material, thereby increasing the useful life of the device. The electrolyte may be liquid, gel, paste or solid. The display devices also include a transparent electrode in contact with the electrochromic material comprising a conductive layer, such as tin oxide, deposited on a transparent glass or plastic substrate.
Japanese Patent No. 56-109317 to Nagasawa et al, published Aug. 29, 1981, discloses electrochromic display devices having a layer of amorphous WO.sub.3 and a layer of crystalline WO.sub.3 separated from each other by an ion conductive layer such as SiO, Al.sub.2 O.sub.3, ZrO.sub.2, MgF.sub.2 or CaF.sub.2. The devices also include a transparent electrode in contact with each of the WO.sub.3 layers comprising tin oxide, indium oxide or indium tin oxide (ITO). The crystalline WO.sub.3 is said to maintain a coulomb balance within the devices and to produce long life and high reliability.
U.S. Pat. No. 4,135,790 issued Jan. 23, 1979 to Takahashi et al discloses electrochromic elements comprising a thin layer of electrochromic material and a thin layer of electron blocking material sandwiched between a pair of transparent electrodes to form a unit cell. Multiple unit cells are stacked together to form a multi-layer structure. The electrochromic material may be WO.sub.3 or MoO.sub.3. The use of multiple thin layers of electrochromic material is said to reduce the response times of the devices.
U.S. Pat. No. 4,768,865 issued Sep. 6, 1988 to Greenberg et al discloses transparent electrochromic windows using WO.sub.3 as the electrochromic material along with a layer of ion conductive material. A counter electrode in the form of a metal grid is placed in contact with the ion conductive material. The metal grid participates in a balancing half-cell reaction whereby the metal grid is oxidized or reduced in response to the electrochromic transition of the WO.sub.3. Use of the metal grid is said to allow operation of the device at lower potentials which prevents electrolysis of water and concurrent gas evolution. The devices have a response time on the order of two minutes. Similar devices are disclosed by Kuo-Chuan Ho, David E. Singleton and Charles B. Greenberg in an article: "Effect of Cell Size on the Performance of Electrochromic Windows," Proceedings of the Symposium on Electrochromic Materials, Proceedings--The Electrochemical Society, Vol. 90, No. 2 pp. 349-364 (1989).
U.S. Pat. No. 4,887,890 issued Dec. 19, 1989 to Scherber et al discloses transparent electrochromic panes or foils including an electrochromic polymer layer and an electrolyte layer sandwiched between two transparent electrodes. Suitable polymers include polyaniline, poly-O-phenyldiamine, polyaniline-3-sulfanic acid, polypyrol and polythiophene, while suitable electrolytes include polymeric sulfonic acid, polymeric carbonic acid, buffered H.sub.2 SO4, buffered HClO.sub.4 and HCl. Suitable transparent electrodes include In.sub.2 O.sub.3 /SnO.sub.2 (ITO), SnO.sub.2, In.sub.2 O.sub.3, Mo, Pd, Pt, Rh, Ti and ZnSe which may be coated on a glass pane or foil. The devices are said to have response times on the order of a few seconds.
U.S. Pat. No. 4,749,260 issued Jun. 7, 1988 to Yang et al discloses transparent electrochromic display devices including a layer of polyaniline electrochromic material and a layer of electrolyte material disposed between two transparent electrodes. The electrodes comprise a transparent conductive coating such as SnO.sub.2, In.sub.2 O.sub.3, Pt or Au deposited on a glass or plastic sheet. The devices may employ multiple layers of electrochromic materials to produce tint and color changes.
U.S. Pat. No. 4,550,982 issued Nov. 5, 1985 to Hirai discloses electrochromic display devices including a layer of electrochromic material and a layer of electrolyte material disposed between two transparent electrodes. The electrochromic material consists of a polymer film comprising at least one organic electrochromic material and at least one ionic material wherein the ionic material is capable of exchanging ions with the organic electrochromic material to serve as an ion donor or acceptor. Suitable electrodes include SnO.sub.2 or ITO coated on a glass or plastic plate. The devices possess a response time on the order of 0.5 to 6 seconds.
In an article by Richard M. Bendert and Dennis A. Corrigan entitled: "Effect of Coprecipitated Metal Ions on the Electrochromic Properties of Nickel Hydroxide," J. Electrochem. Soc., Vol. 136, No. 5, May 1989, pp. 1369-1374, electrochromic films are disclosed comprising nickel hydroxide alone and in combination with minor amounts of other metal hydroxides. The films are said to possess lo high coloration efficiency throughout the visible region and to be useful in window applications.
U.S. Pat. No. 4,263,105 issued Apr. 21, 1981 to Robillard et al discloses electrosensitive recording materials having an electrosensitive layer comprising a bismuth or antimony salt and a semiconductor oxide pigment, such as TiO.sub.2, ZnO, SnO.sub.2 or Al.sub.2 O.sub.3, dispersed in a binder. The electrosensitive layer is deposited on a conductive layer, such as a metal foil, which in turn is laminated on a paper sheet or plastic film substrate. Permanent recordings may be formed by contacting the electrosensitive layer with a writing electrode.
U.S. Pat. No. 4,596,635 issued Jun. 24, 1986 to Warszawski discloses recording media comprising an electrosensitive layer coated on a substrate such as paper or plastic film. The electrosensitive layer comprises a homogeneous aqueous solution of a hydrosoluble salt of a cathodically depositable metal and a hydrosoluble polymer resin. Suitable electrodepositable metals are said to be Zn, Cd, Pb, Ag, Cu, Fe, Ni, Co, Sn, In, Pt, Pd, Au, Bi, Sb, Te, Se, Mn, Tl, Ga, As, Hg and Cr. Suitable polymer resins include hydroxyethylcellulose and carboxymethylcellulose. Permanent recordings may be produced by contacting the electrosensitive layer with a writing electrode.
European Patent Application Publication No. 0300919 of Warszawski, published Jan. 25, 1989, which is hereby incorporated by reference, discloses electrochromic display devices comprising an electrolytic material disposed between two electrodes. The electrolytic material comprises a water soluble salt of a cathodically depositable metal and a watersoluble polymer resin. Suitable cathodically depositable metals are said to include Zn, Cd, Pb, Ag, Cu, Fe, Co, Ni, Sn, In, Pt, Pd, Au, Bi, Sb, Te, Mn, Ti, Se, Ga, As, Hg, Cr, W and Mo, while suitable polymer resins include hydroxyethylcellulose, polyvinyl alcohol and polyvinylpyrrolidone. The electrodes comprise a first transparent working electrode and a second counter electrode which may, or may not, be transparent, depending on the desired application. The working electrode may comprise a thin layer of gold, tin oxide, indium oxide or ITO deposited on a glass or plastic substrate, while the counter electrode may comprise a flexible graphite sheet, a plastic material filled with particles of carbon or metal, or a glass or plastic sheet coated with a thin layer of semiconductor oxide. The disclosed devices are useful for the display of alphanumeric, graphic and other visual information.